The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. A prior art aromatics complex flow scheme has been disclosed by Meyers in Part 2 of the Handbook of Petroleum Refining Processes, Second Edition, 1997, published by McGraw-Hill.
In general, a xylene production facility can have various types of processing reactions. One is a transalkylation in which benzene and/or toluene are reacted with C9+ aromatics to form xylene. Another is xylene isomerization, which may also include dealkylation, where a non-equilibrium mixture of xylenes is isomerized. And another is the disproportionation of toluene to form benzene and xylene.
In the transalkylation process, adverse side reactions can occur. For instance, the aromatic ring may become saturated or even cleaved resulting in naphthene and acyclic paraffin (non-aromatics) co-production. The co-production of these non-aromatics, of course, results in a loss of valuable aromatics. Moreover, benzene is often a sought co-product from a xylene production facility. As some of the non-aromatics have similar boiling points to benzene (benzene co-boilers), they are not readily removed to achieve a benzene product of sought purity for commercial applications which frequently demand a benzene product having at least a 99.85 percent purity.
U.S. Pat. No. 3,562,345 discloses catalysts for transalkylation or disproportionation of alkylaromatics comprising aluminosilicates such as mordenite. Catalytically active metals such as groups VIB and VIII metals may be present.
U.S. Pat. No. 4,857,666 discloses a transalkylation process over mordenite and suggests modifying the mordenite by steam deactivation or incorporating a metal modifier into the catalyst.
U.S. Pat. No. 5,004,855 discloses a catalyst for dealkylating ethylbenzene containing a hydrogenation metal such as platinum, nickel or rhenium and acidic zeolite. They state that the catalyst is subjected to a sulfiding treatment before use. While they state that any method capable of converting rhenium to a sulfide can be adopted for the sulfiding treatment, they prefer sulfiding with hydrogen sulfide at a temperature between room temperature and 500° C. The sulfiding treatment can be carried out in a reaction vessel just before use or before the calcination for activation in air. By sulfiding, the activity of the catalyst is purported to be increased and the loss of xylene due to the side reaction is said to be decreased.
U.S. Pat. No. 5,763,720 discloses a transalkylation process for conversion of C9+ aromatics over a catalyst containing zeolites illustrated in an extensive list including amorphous silica-alumina, MCM-22, ZSM-12, and zeolite beta, where the catalyst further contains a Group VIII metal such as platinum.
U.S. Pat. No. 5,942,651 discloses a transalkylation process in the presence of two zeolite containing catalysts. The first zeolite catalyst is selected from the group consisting of MCM-22, PSH-3, SSZ-25, ZSM-12, and zeolite beta. The second zeolite catalyst contains ZSM-5, and is used to reduce the level of saturated co-boilers in making a higher purity benzene product.
U.S. Pat. No. 5,952,536 discloses a transalkylation process using a catalyst comprising a zeolite selected from the group consisting of SSZ-26, A1-SSZ-33, CIT-1, SSZ-35, and SSZ-44. The catalyst also comprises a mild hydrogenation metal such as nickel or palladium, and can be used to convert aromatics with at least one alkyl group including benzene.
U.S. Pat. No. 5,847,256 discloses a process for producing xylene from a feedstock containing C9 alkylaromatics with ethyl-groups over a catalyst containing a zeolite component that is preferably mordenite and with a metal component that is preferably rhenium.
U.S. Pat. No. 6,060,417 discloses catalysts and processes for transalkylation of alkylaromatics wherein the catalysts comprise mordenite, inorganic oxide and/or clay and at least one metal component of rhenium, platinum and nickel. See also, U.S. Pat. No. 6,359,184.
U.S. Pat. No. 6,867,340 discloses disproportionation/transalkylation catalysts having a carrier and a metal component on the carrier. The metal component is platinum and either tin or lead, and the carriers comprise mordenite and/or beta zeolite with certain Al/Si2 ratios, optionally ZSM-5 zeolite with certain Al/Si2 ratios, and binder. The benefits of the catalyst are said to be high yields of xylenes and preventing catalyst deactivation.
U.S. Pat. No. 6,872,866 discloses a liquid phase xylene isomerization process which uses a zeolite beta and pentasil-type zeolite. The catalyst can contain a hydrogenation metal component such as a platinum group metal and modifiers such as rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof.
US Patent Application Publication No. 2005/0026771, now U.S. Pat. No. 7,202,189, discloses catalysts for transalkylation of C7, C9, and C10 aromatics to C8 aromatics having a trilobe shape with a maximum effective diameter of 0.16 cm. The catalyst is composed of a support, which can be selected from the group consisting of mordenite, beta, MFI, silica-alumina and mixtures thereof. The catalyst is also composed of an optional element deposited on the support selected from the group consisting of platinum, tin, lead, indium, germanium, rhenium, or any combination of these elements. The catalyst also can contain a binder, which is preferably alumina. The preferred support is mordenite.
US Patent Application Publication No. 2005/0266979, now U.S. Pat. No. 7,220,885, discloses catalysts having a sulfur component, a rhenium component, and a solid-acid component for transalkylation processes to convert aromatics into xylenes with decreased methane production. The catalysts have a solid-acid component such as mordenite, mazzite, zeolite beta, ZSM-11, ZSM-12, ZSM-22, ZSM-23, MFI topology zeolite, NES topology zeolite, EU-1, MAPO-36, MAPSO-31, SAPO-5, SAPO-11, SAPO-41, and silica-alumina. The sulfur component may be incorporated into the catalyst by any known technique. Any one or a combination of in situ and/or ex situ sulfur treatment methods is preferred. The resulting catalyst mole ratio of sulfur to rhenium is preferably from about 0.1 to less than about 1.5. In ex situ sulfiding, the catalyst is contacted with a source of sulfur at a temperature ranging from about 0° to about 500° C. The source of sulfur, typically hydrogen sulfide, can be contacted with the catalyst directly or via a carrier gas, typically, an inert gas such as hydrogen or nitrogen. The catalyst composition can also be treated in situ where a source of sulfur is contacted with the catalyst composition by adding it to the hydrocarbon feed stream in a concentration ranging from about 1 ppm-mole sulfur to about 10,000 ppm-mole sulfur. Typical examples of appropriate sources of sulfur include carbon disulfide and alkylsulfides such as methylsulfide, dimethylsulfide, dimethyldisulfide, diethylsulfide and dibutylsulfide.
Mordenite, due to its high transalkylation activity, has found application as a catalyst component for transalkylation processes. The addition of rhenium as a hydrogenation component has greatly enhanced the performance of the catalyst in transalkylation processes. Under transalkylation conditions, ethyl substituents from, e.g., methylethylbenzene, are typically cleaved from the aromatic ring and should be hydrogenated to ethane. One of the problems is that the reaction must be selective. Thus, the hydrogenation should be sufficient to convert an ethylene to ethane yet not result in hydrogenation of the aromatic ring. Heretofore transalkylation catalysts have used relatively small amounts of rhenium, generally up to about 0.2 mass percent, in order to achieve a balance between hydrogenation activity and the avoidance of Ring Loss. The low metal loading, however, results in a catalyst that has a higher deactivation rates than desired, especially with feeds containing aromatics of 10 or more carbons. Feeds containing these higher alkylaromatics are advantageous in order to recover more xylene values from a xylene production unit. Other molecular sieves including MFI have been suggested for transalkylation. Accordingly, a need exists for catalysts and processes for the transalkylation of alkylaromatics, which processes have desirable activities and selectivities of conversion to the desired alkylaromatics such as xylenes, yet result in low Ring Loss, have improved stability, and provide a benzene co-product having a low content of benzene co-boilers, i.e., a low content of non-aromatics having 6 and 7 carbon atoms.